1. Field of the Invention
The present invention relates generally to timer circuits and in particular to adjustable timer circuits which may be implemented completely in integrated circuit form and which is capable of providing relatively long duration timing pulses.
2. Background Art
In many integrated circuits, there is a requirement that certain functions be carried for a relatively precise time period. By way of example, in flash memory systems, the memory cells are programmed and erased by application of certain voltages to the flash memory cells for a fixed duration of time. A typical programming pulse may have a duration on the order of one microsecond. A typical erase pulse may have a duration on the order of one to a hundred milliseconds.
One conventional approach for generating pulses having durations which may vary three or more orders of magnitude is to generate a high frequency clock. This clock is used to produce the short duration programming pulses and a frequency divider circuit is used to divide the clock frequency down to provide the long duration pulses used in erasing. One disadvantage of this approach is that the frequency divider circuit must have a large number of stages to provide the desired long duration output pulses. In addition, many conventional timing circuits capable of being completely implemented in integrated circuit form do not provide the absolute accuracy often necessary in many applications such as flash memory systems.
The present invention pertains to an adjustable timer circuit which may be completely implemented in integrated circuit form and which has a relatively high absolute accuracy. In addition, the subject timer circuit is capable of providing a wide range of timing pulse outputs such as required in flash memory applications without resorting to a large frequency divider circuit. These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following Detailed Description of the Invention together with the drawings.